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* What is SIP?
Posted Feb 19, 2004 - 03:07 PM
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SIP Service Providers SIP is an abreviation of Session Initiation Protocol. It is an internet Standard and as the name allready descibes, it allows to Initiate sessions. These sessions can be simple voice calls or even videoconferencing, or even more advanced.SIP is an official Internet Protocol standard and it is quickly gaining market in becomming the facto VoIP standard.

Ok, now I hear you asking why should I care......

You should care if you use any form of telephone.

As SIP is becomming the standard for VoIP handsets it's better to get a software or hardware phone that supports this standard to ensure future-proof compatability (so far as is possible in any new emerging market).

The SIP protocol allows you to contact people with addresses that are like email addresses. For example you could call me via SIP by dialing from another SIP phone, or SIP PC Client using a soundcard, microphone and speakers.

A SIP address consists of a username and a server name, similar to an email address. In this case tjardick is my username and is the domain which holds the SIP Proxy server.

The SIP Proxy server keeps track of all logged in users. There are several well known Free to use SIP Proxy servers available o­n the Internet, but more about that later.

Because most of us have dynamic IP addresses, it would be extremely difficult to keep track of where your business partners, friends or family are. Luckily, the Proxy server is here to help us. These servers keep track of an "alias" database, allowing us to do what we are used to doing, dialling numbers. Dialing is the same as dialing

Here's what happens when you want to place a call to

  1. Your SIP phone, will contact and ask for a call with tjardick.
  2. The server will check if tjardick is indeed logged in at the moment.
  3. If so it will send a message off to my phone to see if I'm there (e.g. not in busy mode) and tells it that there is a call for me.
  4. A set of messages are sent back and forth between my phone and your phone (some via the server, some direct) to send CallerID information,  make my phone ring and, finally, prepare for a direct voice connection between our two phones.
  5. When i pickup my phone there is a connection and we can talk!
Ok so now you know, in basic form, what SIP does, I'll show some more things that SIP is capable of.

If, for some reason, my phone wasn't logged in, and the SIP Proxy server has been setup to support it, the SIP proxy server could have directed your phone to my voicemail box. Or maybe I was in a call already in which case you would get voicemail too.

SIP telephones (either hard or software) also support the redirect option, so I could have redirected my phone to that of someone else in my absence.

And just like 'normal' phones (I wonder how long will it take before we call SIPphones normal phones?) you can do transfers, conferencing etc. etc. (if your phone supports it).

The nice thing about SIP and the use of a SIP Proxy server is that it doesn't matter where your phone is at that moment. Let's say for example I'm going to spend a couple of days with my Parents in the Netherlands. Broadband internet is available so I take phone with me and plug it in the router at my parents house. My phone will login to the proxy server and say: "hey I'm here". From that moment o­n any of my calls will be available there.

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